For the flow measurement, a floating element, provided with a permanent magnet and arranged inside the measuring tube, is lifted to a greater or lesser extent by the fluid flowing through the measuring tube. For the detection of the position of the floating element, at least one follower magnet is arranged outside the measuring tube in a magnet holder, which is mounted in a rotatable manner about a rotation axis. By means of this assembly, the upward and downward movement of the float inside the measuring tube is converted into a rotational movement that also depends on the flow volume of the fluid in the measuring tube. The movement of the at least one follower magnet or rotational movement caused by its movement is reflected and made apparent to the user by means of a mechanical or electronic indicating element. Such variable-area flow meters are described, for example, in DE 3304615 A1 and DE 3505706 A1.
One problem encountered in the transmission of the linear movement of the float to the indicating device is that the conversion into the rotational movement is not linear over the entire range of the float. Conventionally, when the linear lift of the float is plotted against the rotation angle about the rotation axis in the indicating device, S-shaped curves result (see FIG. 9). For example, if the rotational movement is to be illustrated with a pointer as the indicating element and the flow rate of the fluid is to be indicated on the basis of a corresponding scale, the non-linear conversion of the linear movement into the rotational movement will result in an irregular scale with a small spread. This complicates the reading of the measurement result. In principle, the same applies to an electronic indication of the measurement results since the measurement result to be indicated in this case is conventionally also determined on the basis of the rotation angle.
The indication of the measurement results is further disturbed by the influence of interfering magnetic fields, whether through the Earth's magnetic field or magnetic fields in the vicinity of the variable-area flow meter. These can lead to a situation in which the rotational movement is not exclusively determined by the interaction of the permanent magnet inside the float and the at least one follower magnet of the indicating device, but is instead distorted by external influences.
Accordingly, the invention aims to reduce and ideally completely avoid the problems described above and to achieve a conversion of the upward and downward movement of the float into a rotational movement in the indicating device that is as linear as possible. Moreover, magnetic disturbances should preferably be prevented as far as possible in order to avoid a falsification of the indicated measurement results.